The present invention relates to the fumigation of agricultural products. More particularly, an apparatus and method are provided for removing fumigation gasses from a structure.
Various fumigation methods are used to control pests in agricultural products. In general, a fumigation agent is placed in an agricultural storage structure in order to control any pests present. For example, widely used fumigation agents include aluminum phosphide and magnesium phosphide (both of which release hydrogen phosphide gas). Methyl bromide is also widely used as a fumigation agent. The pests to be controlled are usually insects such as the lesser grain borer and the granary weevil, but also may include rodents. Numerous agricultural products such as grains, rice, beans, and tobacco may be fumigated. The fumigation may occur in a variety of different agriculture storage or transportation structures or facilities including, for example, silos, bins, warehouses, caverns, railroad cars, barges or ships.
Prior to a fumigation, the structure is generally sealed in a gas tight manner to minimize the escape of the fumigation gasses into the atmosphere. In performing a fumigation, a fumigation agent may be introduced to the storage structure in the form of a liquid, solid or gas. Gasiform fumigants released from the fumigation agent then travel throughout the agricultural product and kill the pests. In order to uniformly spread the fumigation gasses, the agricultural product may be mixed as the dose is applied, the fumigation agent may be probed into the product, or the air within the structure may be circulated. For example, U.S. Pat. No. 4,200,657 to Cook discloses a method of recirculating the air within a structure in order to aid the fumigation gas distribution.
Following an appropriate fumigation exposure time, the structure is usually ventilated to remove the fumigation gasses. Traditionally a structure was exhausted by simply opening the structure to allow the gases to escape or be blown into the atmosphere. However, as safety and environmental concerns increased, there arose a need to prevent fumigation gasses from escaping into the atmosphere, and thus methods of "scrubbing" or removing fumigation gasses from exhaust gas streams have been developed. For example, U.S. Pat. No. 4,966,755 to Smith discloses a method of removing fumigation gases from an enclosure. In that method, the gas is pumped through an absorbent chamber that removes any hazardous material prior to exhausting the gas to the atmosphere. Likewise, U.S. Pat. No. 4,812,291 to Friemel et al. discloses a method of removing hydrogen phosphide from a storage structure after fumigation. In Friemel, air from a fumigated structure is blown by a fan through beds containing a hydrogen phosphide binding and decomposition catalyst. Upon passing through the beds, the air is exhausted to the atmosphere.
Fumigation scrubbing methods such as disclosed by Smith and Friemel et al. generally entail a single pass of the air through an adsorbent bed prior to exhausting the air to the atmosphere. Thus, such systems remove substantially all of the fumigation gas in a single pass through adsorption beds and such methods generally use a large amount of adsorbent material and large fans or pumps for each adsorption unit. Thus, the costs of each adsorption unit may be high and the size large. It is desirable to minimize both the costs and size and to create a portable system.